[What are the unique characteristics of optico-spinal MS in Japanese and neuromyelitis optica in western populations].

Although the diagnosis of optico-spinal MS (OSMS) is solely based on the unique lesion distribution, the OSMS is clinically characterized by distinctive features which are mostly shared by the clinical characteristics of relapsing neuromyelitis optica (NMO). Conversely, Western investigators appear to consider relapsing NMO as a distinct entity from MS, and distinct characteristics and independent diagnostic criteria for NMO were proposed. However, the key characteristics of OSMS and NMO seem to be quite similar, and therefore the prototype of these disorders are identical which we would like to call OSMS/NMO. Most of the described characteristics of the OSMS/NMO appear to reflect the fulminant nature of each attack and the expansion of each lesion, which we called "attack-related severity". Recently, we found that clinical characteristics seem to be distinctively different between OSMS patients with and without longitudinally extending spinal cord lesions while clinical features of OSMS patients without extending spinal cord lesions are similar to those of CMS patients. To understand the pathomechanisms of OSMS and NMO, the "attack-related severity" must be an important key factor as well as the unique lesion distribution.